Apparatus and method for producing single crystal structures



March 22, 1955 R s 3,242,015

APPARATUS AND METHOD FOR PRODUCING SINGLE CRYSTAL STRUCTURES Filed Sept.24, 1963 4 Sheets-Sheet 1 FIG. 1

H I: I 6/ J \7V k\\5 7\ J" a 4 4A FIGZ FIG 3 I5 INVENTOR.

DARREL M, HAR RIS ATTORNEY March 22, 1966 M HARRIS 3,242,015

APPARATUS AND METHOD FOR PRODUCING SINGLE CRYSTAL STRUCTURES Filed Sept.24, 1963 4 SheetsSheet 2 INVENTOR DARREL M. HARRIS BY @M}.M

ATTORNEY March 22, 1966 A s 3,242,015

APPARATUS AND METHOD FOR PRODUCING SINGLE CRYSTAL STRUCTURES Filed Sept.24, 1963 4 Sheets-Sheet 5 TEMPERATURE vs 7a OF FURNACE LENGTH A I250 U LLLI Q: 3 Z 1240 0: I238 LL] 0.. E a

l l 0 20 4o 60 80 I00 To OF FURNACE LENGTH INVENTOR FURNACE GRADIENTDARREL HARR'S -INGOT GRADIENT BY ATTORNEY March 22, 1966 HARRls3,242,015

APPARATUS AND METHOD FOR PRODUCING SINGLE CRYSTAL STRUCTURES Filed Sept.24, 1963 4 Sheets-Sheet 4 TEMPERATURE VS 75 OF FURNACE LENGTH H I250 U oLIJ Q: a 1240 7 I238 LJJ Cl. 2 LL] I230 IZIO 0 2O 4O 6O 80 I00 7o OFFURNACE LENGTH INVENTOR, FURNACE GRADIENT D R HARR|5 ----THERMAL SHIELDGRADIENT BY g /L INGOT GRADIENT ATTORNEY United States Patent C)3,242,015 APPARATUS AND METHOD FOR PRODUCING SINGLE CRYSTAL STRUCTURESDarrel M. Harris, Glendale, Mo., assignor to Monsanto Company, acorporation of Delaware lFiled Sept. 24, 1963, Ser. No. 311,258 9Claims. (Cl. 1481.6)

This invention relates in general to certain new and useful improvementsin single crystal elements and compounds and more particularly, tosingle crystal elements and compounds which are formed by thetemperature gradient freeze process.

Today, single crystal substances have found widespread application inthe electronics industry, for use in the manufacturing of semi-conductordevices such as transistors and rectifiers. The operation andperformance of these semiconductor devices, however, is primarily basedupon the properties of the single crystal structure. Consequently, theamount of crystal defects appearing in the structure of the crystal mustbe relatively small before the structure is suitable for use insemiconductor devices. Therefore, it is of strategic importance toproduce a single crystal structure which is relatively pure and which isrelatively free of cracks and crystal dislocations. A crystaldislocation sets up internal strains within the crystal which willeventually initiate undesirable polycrystalline growth.

One rather effective method, heretofore used in the production of singlecrystal elements and compounds, is the temperature gradient freezemethod. This method generally consists of placing polycrystallinematerial in a crucible, melting the polycrystalline material in thecrucible, and placing the crucible in a tube furnace which is capable ofproducing a temperature gradient along its length so that it is hotterat one end than at the other. As the temperature of the furnace isreduced, and the gradient is shifted, a portion of the material withinthe crucible will freeze causing a solid-liquid interface. Thus, whenthe gradient has shifted to a point below the freezing point of thematerial, a single crystal structure is formed within the crucible.

While the temperature gradient freeze method has been effective, it hassuffered certain serious deficiencies. In the first place, it is ratherdilficult tomaintain a linear temperature gradient across the entirelength of the crucible. Secondly, it has been difficult to maintain evenheating distribution across the entire length of the crucible. Third'ly,it has been rather difficult to control the solid-liquid interface.Recent investigations have shown that an accurate solid-liquid interfacematerially reduces the possibility of crystal dislocation, imperfectionssuch as microscopic cracks and uneven crystal growth. However, therehave been no effective methods devised for effectively controlling theshape of the interface.

It is, therefore, the primary object of the present invention to providean apparatus and method for producing single crystal elements andcompounds having a high degree of crystal purity and excellent crystalstructure.

It is a further object of the present invention to provide an apparatusand method for controlling the liquidsolid interface when single crystalstructures are produced by the temperature gradient freeze method.

It is another object of the present invention to provide an apparatus ofthe type stated which is cap-able of maintaining a relatively uniformtemperature gradient along the longitudinal axis of the crystal bearingcontainer.

It is an addition-a1 object of the present invention to provide anapparatus of the type stated which is relatively inexpensive tomanufacture.

It is another salient object of the present invention to provide a newand simple method for producing large amounts of single crystalstructure by the temperature gradient freeze process.

With the above and other objects in view, my invention resides in thenovel features of form, construction, arrangement, and combination ofparts presently described.

In the accompanying drawings:

FIGURE 1 is a front elevational view of an apparatus for producingsingle crystal structures and which is constructed in accordance withand embodying the present invention;

FIGURE 2 is a horizontal sectional view taken along line 22 of FIGURE 1;

FIGURE 3 is a longitudinal sectional view taken along line 3-3 of FIGURE2;

FIGURE 4 is a transverse sectional view taken along line 4-4 of FIGURE2;

FIGURE 5 is a top plan view of a modified form of an apparatus forproducing single crystal structures and which is constructed inaccordance with and embodying the present invention;

FIGURE 6 is a vertical sectional view taken along line 66 of FIGURE 5;

FIGURE 7 is a diagrammatical view of a temperature gradient freeze chartshowing the temperature gradient over a length of ingot when the ingotwas cooled in accordance with prior art methods; and

FIGURE 8 is a diagrammatical view of a temperature gradient freeze chartshowing a temperature gradient over a length of ingot when the device ofthe present invention is employed.

Generally speaking, the present invention comprises a heat conductingshield and a heat insulating shield which are concentrically disposedabout a crucible holding the polycrystalline material. The device isthen placed in a temperature gradient freeze furnace for heating thepolycrystalline material above its melting point to produce a liquid orso-called melt. The melt is then cooled in such a manner that aliquid-solid interface is formed and moves from one end of the crucibleto the other. Through the combination of heat conducting andheatinsulating shields, it is possible to obtain relatively linear heatapplication to the entire length of the crucible and hence maintain goodcrystal structure with relatively few crystal dislocations.

Referring now in more detail and by reference characters to the drawingswhich illustrate practical embodiments of the present invention, Adesignates an apparatus for producing single crystal structures whichcomprises an elongated boat-shaped crucible or so-called crystallizingcontainer 1, preferably made of quartz, and which is often referred toin the art as a boat. The boat 1 includes a bottom wall 2 whichintegrally merges into a pair of sidewalls 3, 4 and which are in turnintegrally connected by transverse end walls 5, 6.

The boat 1 is then suitably placed in an open ended transparent ampule'7 which is then sealed in any conventional manner such as by fusing aremovable end 8 thereto. The ampule 7, however, is conventional in itsconstruction, and is therefore not described in detail herein.

The ampule 7 is concentrically disposed within an opended tubular heatconducting sleeve 9, preferably formed of silicon carbide or other heatconducting material which has a higher heat conductivity than thecrucible 1 and the material being treated in the crucible 1. The heatconducting sleeve 9 is preferably formed with a wall thickness ofapproximately inch for a 12 inch length, the wall thickness increasingwith an increase in length. A tubular heat insulating sleeve or heatinsulating shield preferably formed of fibrous aluminum silicate, andhaving a heat conductivity of 2.26 B.t.u./hr./sq. ft.- F. at normaloperating temperature of 1250 F., is concentrically disposed around andextends axially along the heat conducting sleeve 9. At thesetemperatures, the shield 10 is preferably formed of a fibrous materialsince the insulating qualities thereof are improved. The heat insulatingshield 10 is preferably circular in vertical cross-section, and has anannular wall thickness of approximately inch, for a 10 inch length, thewall thickness also increasing with an increase in length. Moreover, byreference to FIGURES 2 and 3, it can be seen that the heat insulatingshield 10 is slightly shorter in overall length than the heat conductingsleeve 9, thereby defining an extended heat gathering terminal portion11 on the heat conducting sleve 9 which preferably has a length of 2inches. At its left transverse end, reference being made to FIGURE 2,the heat insulating shield 10" is integrally provided with an inwardlystruck annular flange 12 having a radial thickness which isapproximately equal to the annular wall thickness of the heat conductingsleeve 9 and thereby forms a central heat dissipating aperture 13 whichis aligned with the bore of the heat conducting sleeve 9. A heatinsulating plug 14 preferably formed of the same material as the shield10, is removably disposed in the open end of the heat gathering portion11 in a closure-wise position. Fibrous calcium aluminate has also beenfound to be very suitable as a material of construction for the shield10. When low melting point materials are used in the crucible 1, a metalsleeve, such as nickel, tungsten or molybdenum can be employed for theshield 10. Use of a metal of this type will give a greater heatconductivity than the material in the crucible 1 and the shield 10 will,if maintained in a reducing atmosphere, exhibit a bright heat refleetingsurface and thereby reduce radiation from the sidewalls of thecrucible 1. For this purpose, it is desirable to inject an inert orreducing gas such as argon, or hydrogen into the furnace 15.

The aforementioned assembled components are then placed in a suitablefurnace 15 which is capable of producing single crystal structures bythe temperature gradient freeze process. The furnace 15 is provided witha pair of upstanding supports 16 which engage the shield 10 near each ofits transverse ends. The remainder of the furnace 15 is conventional inits construction, and is therefore not described in detail herein.However, the furnace 15 is provided with a heating element 17 which isdesigned to cause a linear temperature differential between the two endsof the furnace 15. While the furnace 15 illustrated herein is providedwith a heating element 17 with progressively increased spacing betweenthe convolutions thereof, it should be understood that any suitablefurnace such as the so-called Globar or induction furnaces could beemployed.

In operation, the crucible 1 is initially charged with a suitable amountof polycrystalline material, such as gallium arsenide, to theapproximate level as shown in FIG- URE 3. The crucible 1 is next placedwithin the ampule 7, the removable end 8 is sealed, the ampule 7 isevacuated and is in turn placed within the heat conducting sleeve 9. Byreference to FIGURE 3, it can be seen that the heat conducting sleeve 9is slightly longer than the overall length of the ampule 7, and moreoverhas its terminal portion 11 located within the hot end of the furnace15. Next, the heat conducting sleeve 9 is concentrically disposed withinthe heat insulating shield 10 until the left transverse end of thesleeve 9 abuts the interior surface of the annular flange 12,substantially as shown in FIG- URE 3. Finally, the assembled componentsare suitably placed within the furnace 15. By reference to- FIGURE 1, itcan be seen that the greater number of turns of the heating element 17are located near the right transverse end of the furnace 15, and hencethis is the hotter portion of the furnace 15. It can be seen that thespacing between the convolutions of the heating coil or heating element17 increases as it traverses the length of the furnace 15. Consequentlythe left transverse end of the furnace 15 is cooler than the righttransverse end, causing a temperature differential through out thelength of the furnace, which differential is relatively linear.

The heating coil 17 is ultimately connected to a suitable source ofelectrical current (not shown), and through suitable control means (alsonot shown) is heated to a temperature above the melting point of thegallium arsenide so that the entire charge melts. The crucible 1 and themelt are then cooled so that freezing begins at the left transverse endof the crucible 1 or at the transverse end wall 6. Further cooling iscarried out in the furnace 15, while still maintaining the lineartemperature differential throughout the entire length thereof, so thatan isothermal surface or liquid-solid interface s near the melting pointpasses progressively through the melt until the entire melt hassolidified. In this manner a single nucleus which first forms near theend wall 6 in the crucible 1 can be made to grow and fill the entirecrucible, yielding a single crystal approximately of the size and shapeof the crucible itself.

The temperature of the furnace 15 is maintained at a point so that thesolidification takes place at a relatively slow rate. In connection withthe above, a rate of solidification where the interface s moves atapproximately 0.1 to 0.3 centimeter per hour has been found to producemost desirable results. At this rate, dislocation in the crystal ismaterially reduced. In the past, it was difficult to maintain arelatively constant linear temperature differential across the entirelength of the crucible 2. However, with the heat conducting sleeve 9,the terminal portion 11 absorbs the heat from the hot end of the furnace15 and distributes the heat evenly across the length of the ampule 7 andthe boat 1. Therefore, through the use of the sleeve 9 it has beenpossible to materially reduce crystal dislocations to a point where theyno longer have the tendency to produce polycrystalline growth. In fact,actual tests have shown that through the above described procedures,dislocations in the single crystal structure have been reduced from 10dislocations per square centimeter to 10 dislocations per squarecentimeter.

As it was pointed out above, it is desirable to maintain an arcuateinterface which is concave into the liquid phase, that is to say theinterface lies in an arcuate plane and extends into the liquid phase atthe isothermal surface between the liquid and solid phases. It has beenwell established that this type of interface will materially reducecrystal dislocations and substantially reduce uneven crystal growth.However, in the past, it was rather difficult, if not impossible, tocontrol the shape of the interface using the gradient freeze method ofproducing single crystal structures. In the past, a great deal of heatwas radiated from the side walls of the ampule and boat to the furnace,especially in the colder end thereof. Since the boat and contentsusually have a greater heat conductivity than the media surrounding theboat in the colder end of the furnace, the boat and its contents were ata higher temperature than the media surrounding the boat in the colderend of the furnace. Moreover, the temperature differential between thecold end of the furnace and the portion of the boat located in such endof the furnace Was quite large. These conditions resulted in a greatdeal of heat radiation from the material contained in the boat throughthe side Walls of the boat to the colder end of the furnace and hence,it was quite difficult to maintain the arcuate interface. Moreover,

since the melt did not have a uniformly linear temperature gradientacross its length, some portions of the melt radiated greater amounts ofheat than other portions thereof. The heat insulating shield 10 in thepresent invention, substantially reduced the amount of heat radiationfrom the side walls of the ampule 7 and boat 1. Moreover, since the heatconducting sleeve 9 surrounds the ampule 7 and has a higher heatconductivity than the ampule 7, the boat 1 is located in a media whichis hotter than itself. In effect, this maintains the side walls of theboat 1 in a state of thermal isolation and the boat 1 is in effect thena thermally floating tube. The only heat received by the boat l is thatheat which is conducted by the heat conducting sleeve 9. Furthermore,the only heat loss is through the left transverse end of sleeve 9,reference being made to FIGURE 1, to the colder end of the furnace IS.

The heat which is absorbed by the boat 1 moves towards the center of themelt and of the solidified portion thereof and is added to the heat ofsolidification which passes across the isothermal surface s and outthrough the left rtansverse end of the boat I and heat conducting sleeve9. The heat which is impressed across the walls of the boat 1 in thearea of the isothermal surface s is small compared to the heat ofsolidification passing through the center of the isothermal surface s.Consequently, this additional heat applied along the walls 3, 4 of thecrucible 1 at the isothermal surface s will tend to move aong the walls3, 4 before moving toward the center of the solidified mass in combiningwith the heat of fusion. Similarly, the heat applied at the base 2 ofthe crucible 1 will move along the base 2 at the isothermal surfacebefore combining with the heat of solidification. This additional heatwhich is impressed across the melt adjacent to the isothermal surface swill cause a very slight temperature differential across the portion ofthe melt which faces the isothermal surface s so that the isothermalsurface s will in effect form an arcuate face as shown in FIGURES 2 and3. Thus, as the temperature of the furnace 15 has been lowered Whilemaintaining the same temperature gradient between the two ends thereof,the crucible will have been progressively cooled from the lefttransverse end to the right transverse end thereby shifting theisothermal surface s until the entire melt has completely solidified.

While the operation of the present apparatus has been described in theproduction of a gallium arsenide crystal, it should be understood thatthe invention is not limited to this particular compound. Contemplatedfor use in the present invention is the production of large singlecrystal compounds formed from elements of Groups IIIB and VB of thePeriodic System (Hubbards Chart of the Atoms). The compounds includedwithin this group which are contemplated for use in the presentinvention include arsenides, phosphides and antimonides of aluminum,gallium and indium. It is also contemplated that compounds formed fromthe elements of Groups II and IV, and Groups I and VII of the PeriodicSystem could be used in connection with the present invention. Theseelements include the sulfides, selenides and telluridesof zinc, cadmiumand mercury and the chlorides, bromides and iodides of sodium,potassium, copper, rubidium, silver, cesuim and gold. It should also beunderstood that the apparatus of the present invention could besuccessfully employed for the production of single crystal elements suchas selenium, tellurium, rubidium, germanium, silicon, cesium, gold,silver, etc. However, the heat conducting sleeve employed must have ahigher heat conductivity than the element being crystallized.

It is possible to provide a modified form of Apparatus B for producingsingle crystal structures in accordance with the present invention,substantially as shown in FIGURES 5 and 6. The Apparatus B for producingsingle crystal structures is substantially similar to the Apparatus Aand consists of an elongated boat shaped crucible 18 which is suitablyplaced in an open end transparent ampule 19, both of which aresubstantially identical to the previously described crucible 1 andampule 7 respectively. The ampule 19 is similarly disposed within a heatconducting sleeve 20, which is, in turn, concentrically disposed withina heat insulating shield 21, the sleeve 26 also being provided with aheat insulating plug 22, which are substantially identical to thepreviously described sleeve 9, shield 10 and plug 14 respectively. TheApparatus B is similarly placed within a furnace 23, which is alsoidentical to the previously described furnace 15.

The ampule 19 is not sealed in the manner as previously described in theApparatus A, but is connected to a similar ampule 24 also preferablyformed of a transparent quartz material, through an elongated neck 25and is therefore in communication with the ampule 24. The ampule 24 isplaced within a suitable furnace 26, which is provided with a pair ofupstanding supports 27 for supporting the ampule 24 near each of itstransverse ends. The furnace 26 is conventional in its construction andis therefore not described in detail herein. However, the furnace isprovided with a heating element 28, which has uniformly spacedconvolutions and thereby maintains a linear temperature across thefurnace. The furnace 26 should be capable of producing a temperature ofat least 700 C. An annular heat insulating ring 29 is disposed aroundthe elongated neck 25 and is located in endwise abutment with each ofthe endwise aligned furnaces 23 and 26, in order to maintain the neck 25at the same temperature as the interior of the furnace 26. The ring 29is preferably formed of firebrick or similar refractory material.

The Apparatus B is employed where it is desired to react elementalgallium contained within the crucible 18 with arsenic gas containedwithin the ampule 24. In use, a suitable charge of liquid gallium isdisposed within the crucible l8 and a suitable charge of solid arsenicis placed in the ampule 24. The crucible I8 is placed in the ampule 19and the ampule 119 is sealed in communication with the ampule 24 throughthe elongated neck 25. The ampules 19 and 24 are then evacuated toapproximately 10 mm. of mercury pressure through a tube (not shown) onthe ampule 24, which is ultimately sealed and thereby maintains thevacuum in each of the ampules. The furnace 26 is then heated to atemperature above the melting point of gallium arsenide or 1245 C. andthe furnace 26 is heated to a temperature of approximately 610 to 620C., thereby creating a gaseous arsenic atmosphere. The gaseous arseniccontained Within the ampule 24 will pass through the elongated neck 25and react with the liquid gallium contained within the crucible 18 atthese temperatures to form gallium arsenide.

The crucible l8 and the melt contained therein is then cooled so thatfreezing begins at the left transverse end of the crucible 18 in thesame manner as in the Apparatus A. Further cooling is carried out in thefurnace 23 while maintaining the linear temperature differentialthroughout so that an isothermal surface or liquid-solid interface swhich is similar to the previously formed liquid'solid interface 5,passes progressively through the melt until the entire melt hassolidified. In this manner, it is possible to form a single nucleus ofgallium arsenide by reacting the arsenic gas with the liquid gallium inthe crucible 18 so that the nucleus can grow and fill the entirecrucible yielding a single crystal of gallium arsenide, which isapproximately the size and shape of the crucible itself. It should alsobe understood that it is possible to form single crystal compounds otherthan gallium arsenide by employing a gas other than arsenic and a solidmaterial other than gallium.

Contemplated for use in this embodiment is the production of largesingle crystal compounds formed from the elements of Groups 3B and 5B ofthe Periodic System. Also contemplated for use in this embodiment arecompounds formed from the elements of Groups 2 and 4 and Groups 1 and 7of the Periodic System.

FIGURE 5 is a temperature gradient freeze chart plotting the temperaturein degrees centigrade vs. the furnace length and showing the temperaturegradient across the length of the furnace and the temperature gradientacross the length of the crucible containing a polycrystalline galliumarsenide ingot for the production'of single crystal gallium arsenidestructures. FIGURE 6 is a temperature gradient freeze chart plotting thetemperature in degrees C. vs. the furnace length for the same crucibleused in FIGURE 5, when the heat conducting shield and heat conductingsleeve forming part of the present invention are thus employed.Comparing FIGURE to FIGURE 6, it can be seen, that the temperaturegradient across the length of the crucible very nearly approaches thetemperature gradient across the length of the furnace and has almost thesame slope thereof. However, with reference to FIGURE 6, when the heatconducting sleeve and heat insulating shield are employed, it can beseen that there is a greater disparity between the temperature gradientof the furnace and that of the crucible. Moreover, it can be seen thatthe crucible is in effect surrounded by a continually hotter surfacewhen the heat conducting sleeve is employed. It is also to be noted,that when the heat conducting sleeve and the heat insulating shield areemployed, an almost perfectly smooth temperature differential whichapproaches almost perfect linearity exists across the length of thecrucible I. In other words, with the present invention, there is analmost completely symmetrical temperature environment in the crystallinesystem and hence, the resulting crystal structure produced shows nopolycrystallization of the melt. The data employed to produce thetemperature gradient freeze charts of FIGURES 5 and 6 is more fullydescribed in Examples 1 and 2, hereinafter set forth.

A more detailed description of the invention is set forth in thefollowing examples which have reference to the foregoing specificationand the accompanying drawings:

Example 1 Approximately 450 grams of surface cleaned gallium arsenide isplaced in a quartz crucible having a length of approximately 5 inchesand a diametral cross section of approximately 33 mm. The crucible andits contents are then placed in a quartz ampule having a length ofapproximately 8 inches and a diametral cross section of 38 mm. Theampule is heated to 150 C., then subjected to a high vacuum andevacuated to a pressure of approximately 5 10 mm. of mercury, and isthen sealed. The ampule is then cleaned and placed in a silicon carbidetube having a length of 10 inches and a diametral cross section of 2inches, and a wall thickness of inch. The ampule is axially centeredwithin the silicon carbide tube and located in such a position that onetransverse end of the crucible is located approximately 1 inchinternally of the transverse end of the silicon carbide tube which islocated in the cold end of the furnace. The carbide tube is then wrappedin calcium aluminate paper commercially known as Fiberfrax, until athickness of inch is attained. The silicon carbide tube is then pluggedwith a sufficient amount of the Fiberfrax paper at the hot end thereofin order to prevent direct heating of the ampule in the cruciblecontained within the silicon carbide tube.

The aforementioned assembly is then placed in a temperature gradientfreeze furnace and disposed upon supports within the furnace, which makea minimum physical contact with the insulated tube. The furnace is sizedso that there is a V; inch clearance between the furnace wall and theinsulated silicon carbide tube. Thermo couples are so placed in thesilicon carbide tube that temperatures can be obtained for each inch ofingot length. The furnace is then heated until the coolest end of theingot has reached a minimum temperature of 1242 C., the melting point ofthe charge, and the furnace is maintained at this temperature for atleast 1 hour in order to assure complete melting thereof. Thetemperature of the furnace is reduced over a period of 18 hours 8 untilthe hottest portion of the charge in the crucible is below the meltingpoint of gallium arsenide or 1238 C. The data thus obtained is used inthe production of the temperature gradient freeze chart of FIGURE 6.

Approximately 425 grams of a single crystal of gallium arsenide is thusobtained. Oriented slices of the single crystal structure on the 1l1plane (Miller indices) are removed from the end of the crystal whichfirst froze in the crucible. Similarly, oriented slices are removed onthe same lll plane (Miller indices) from the end of the crystal whichwas located in the hotter portion of the furnace. These oriented slicesare then mechanically polished and treated with a polish etch consistingof 4 parts of water, 3 parts of concentrated nitric acid, and 1 part of48% hydrochloric acid in order to remove the mechanically workedsurfaces thereof. The polished slices are then treated with an etchconsisting of 2 parts of Water and 1 part of concentrated nitric acid todevelop etch pits. The slices from the end of the crucible which waslocated in the colder portion of the furnace are examined under amicroscope and found to contain 1500 etch pits per square centimeter.The slices from the crystal which was located in the hotter portion ofthe furnace are similarly examined and found to contain 6100 etch pitsper square centimeter.

Example 2 The above process used in Example 1 is repeated withapproximately 450 grams of surface clean gallium arsenide in the samequartz boat. However, in this example, the silicon carbide tube and thecalcium aluminate insulating paper is not employed. The data thusobtained is used in the production of the temperature gradient freezechart of FIGURE 5.

Oriented slices thus removed from the end of the single crystal locatedin the colder end of the furnace, in the same manner as in the aboveexample when examined are found to contain approximately 15,000 etchpits per square centimeter. Oriented slices removed from the end of thecrystal which was located in the hotter portion of the furnace, whenexamined are found to contain approximately 65,000 etch pits per squarecentimeter.

Example 3 Example 4 Approximately 220 grams of silicon is charged intothe crucible used in Example 1, and the procedure of Example 2 isfollowed. Thus, in this case the silicon carbide tube and the calciumaluminate paper are not employed. Oriented slices removed from the endof the crystal located in the colder portion of the furnace revealapproximately 100,000 etch pits per square centimeter whereas orientedslices removed from the end of the crystal located in the hotter portionof the furnace reveal approximately 500,000 etch pits per squarecentimeter.

Example 5 Approximately 400 grams of germanium are charged into thecrucible used in Example 1 and the procedure of Example 1 is followed.In this case, the silicon carbide tube and the calcium aluminate paperare disposed around the crucible containing the germanium. Orientedslices removed from the end of the crystal located in the colder portionof the furnace reveal approximately etch pits per square centimeter andoriented slices removed from the end of the crystal located in thehotter portion of the furnace reveal approximately 650 etch pits persquare centimeter.

Example 6 Approximately 400 grams of germanium are charged into thecrucible used in Example 1 and the procedure of Example 2 is followed.In this case, the silicon carbide tube and the calcium aluminate paperare not employed. Oriented slices removed from the end of the crystallocated in the cooler portion of the furnace reveal approx imately 750etch pits per square centimeter, and oriented slices removed from theend of the crystal located in the hotter portion of the furnace revealapproximately 3050 etch pits per square centimeter.

It should be understood that changes and modifications in the form,construction, arrangement and combination of parts presently describedand pointed out can be made and substituted for those herein shownwithout departing from the nature and principal of my invention.

Having thus described my invention what I desire to claim and secure byLetters Patent is:

1. An apparatus for producing single crystal substances by thetemperature gradient freeze method which comprises in combination, acrystallizing container, a heat conducting element surrounding saidcontainer and being in heat exchange relation thereto, a heat insulatingmember surrounding said heat conducting element to prevent heatradiation from the longitudinal surface of said crystallizing container,and heating means surrounding said heat conducting element to apply auniform temperature gradient across said crystallizing container, saidheat conducting element having a terminal end which extends beyond onetransverse end of the heat insulating member, the terminal end of saidelement having a length sufficient to draw heat from the heating meansfor transmitting the heat across the length of the heat conductingelement.

2. An apparatus for producing single crystal substances by thetemperature gradient freeze method which comprises in combination, acrystallizing container located in a temperature gradient atmosphereWhere one end thereof is at a higher temperature than the other end ofsaid container, an open ended heat conducting element surrounding saidcontainer and being in heat exchange relation thereto, a heat insulatingmember surrounding said heat conducting element to prevent heatradiation from the longitudinal surface of said crystallizing container,said heat insulating member being of sufiicient length to surround thecrystallizing container for its entire length, a heat insulating plugdisposed within the open end of the heat conducting element which isproximate to the end of the crystallizing container at the highertemperature and is of sufiicient thickness to prevent direct heatingthrough the open end of the heat conducting element, and heating meanssurrounding said heat conducting elernent to apply a uniform temperaturegradient across said crystallizing container.

3. An apparatus for producing single crystal substances which comprisesin combination, a crystallizing container, a heat conducting elementsurrounding said container and being in heat exchange relation thereto,a heat insulating member surrounding said heat conducting element toprevent heat radiation from the longitudinal surface of saidcrystallizing container, heating means surrounding said heat conductingelement to apply a uniform temperature gradient across saidcrystallizing container, and means in heat conductive association withsaid heat conducting element for drawing heat from the hot end of theheating means and transmitting the heat along the entire length of theheat conducting element, said last named means being located at thehotter end of said gradient and having suflicient surface area to drawthe required amount of heat for transmission along the length of theheat conducting element.

4. An apparatus for producing single crystal substances which comprisesin combination, .a crystallizing container, a heat conducting elementsurrounding said container and being in heat exchange relation thereto,a heat insulating member surrounding said heat conducting element toprevent heat radiation from the longitudinal surface of saidcrystallizing container, and heating means surrounding said heatconducting element to apply a uniform temperature gradient across saidcrystallizing container, said heat conducting element having a greaterlength than the heat insulating member and having a portion whichextends beyond one end of the heat insulating member, said portion beinglocated at the hotter end of said temperature gradient and having alength suflicient to draw heat from the heating means for transmittingthe heat along the length of the heat conducting element.

5. An apparatus for producing single crystal substances which comprisesin combination, a crystallizing container, a heat conducting elementsurrounding said container and being in heat exchange relation thereto,a heat insulating member surrounding said heat conducting element toprevent heat radiation from the longitudinal surface of saidcrystallizing container, and heating means surrounding said heatconducting element to apply a uniform temperature gradient across saidcrystallizing container, said heat conducting element having a greaterlength than the heat insulating member and having a portion whichextends beyond one end of the heat insulating member, said last namedportion extending into the hotter end of the temperature gradient andhaving a length suflicient to draw heat from the heating means fortransmitting the heat along the length of the heat conducting element,said heat insulating member having an annular flange which engages onetransverse end of the heat conducting element.

6. A process for the production of single crystal substances by thetemperature gradient freeze method which comprises melting apolycrystalline form of such substance in a container disposed within acrystallizing zone to produce a melt, adjusting the temperature withinsaid crystallizing zone, to provide a substantially linear temperaturegradient across the entire length of the polycrystalline form of suchsubstance, cooling said crystallizing zone incrementally from one end ata slow uniform rate to initiate crystallization of said melt, therebyforming a crystal in the cooled portion of the crystallizing zone,continually applying heat to the walls of the container along its entirelength but at the substantially linear temperature gradient so that heatflows through the contamer from the hotter end thereof to the colder endthereof, applying heat to the walls of said container at a rate which isless than the rate of movement of heat through the center of the crystalto maintain an arcuate interface between the crystal and the melt, witha temperature differential thereacross, said arcuate interface bemgconcave with the liquid phase of the melt, continually cooling saidcrystallizing zone until the entire melt has crystallized, andrecovering a crystal from the container.

7. An apparatus for producing single crystal substances which comprisesin combination a crystallizing container, a first reacting element insaid crystallizing container, 21 reactant container containing a secondreacting element, and being 111 communication with the reacting elementin said crystallizing container, 21 heat conduct-ing element surroundingsaid crystallizing container and being in heat exchange relationthereto, a heat insulating member surrounding said heat conduct-ingelement to prevent radiatron from the longitudinal surface of saidcrystallizing heat conducting element having a portion beyond one end ofsaid heat insulating member, said last named portion having a lengthwhich is sufficient to gather heat and transmit heat along the length ofthe heat conducting element, first heating means surrounding said heatconducting element to apply a uniform temperature gradient across saidcrystallizing container, and second heating means surrounding saidreactant container.

8. An apparatus for producing single crystal substances by thetemperature gradient freeze method which comprises in combination, acrystallizing container located in a temperature gradient atmospherewhere one end thereof is at a higher temperature than the other end ofsaid container, an open ended heat conducting element surrounding saidcontainer and being in heat exchange relation thereto, a heat insulatingmember surrounding said heat conducting element to prevent heatradiation from the longitudinal surface of said crystallizing container,said heat insulating member being of sufficient length to surround thecrystallizing container for its entire length, a heat insulating plugdisposed within the open end of the heat conducting element which isproximate to the end of the crystallizing container at the highertemperature, said heat insulating member having an annular flange whichengages the transverse end of the heat conducting element which isproximate to the end of the crystallizing container at the lowertemperature, and heating means surrounding said heat conducting elementto apply a uniform temperature gradient across said crystallizingcontainer.

9. A process for the production of single crystal substances by thetemperature gradient freeze method which comprises melting apolycrystalline form of such substance in a container disposed withinsaid crystallizing zone to produce a melt, gathering heat from thehotter end of the temperature gradient and conducting the heat along aheat conductive element surrounding the crystallizing zone, transmittingthe heat by radiation to the container within the crystallizing zone,preventing re-radiation of the heat from the container and radiation ofheat from the conductive element by an insulating element to provide asubstantially linear temperature gradient across the entire length ofthe polycrystalline form of such substance, cooling said crysta-llizingzone incrementally from one end at a slow uniform rate to initiatecrystallization of said melt, thereby forming a crystal in the cooledportion of the crystallizing zone, continually applying heat to thewalls of the container along its entire length but at the substantiallylinear temperature gradient so that heat flows through the containerfrom the hotter end thereof to the colder end thereof, applying heat tothe walls of said container at a rate which is less than the rate ofmovement of heat through the center of the crystal to maintain anarcuate interface between the crystal and the melt with a temperaturedifferential thereacross, said arcuate interface being concave with theliquid phase of the melt, continually cooling said crystallizing zoneuntil the entire melt has crystallized, and recovering a crystal fromthe container.

References Cited by the Examiner UNITED STATES PATENTS 2,475,810 7/1949Theuerer 148-15 2,789,039 4/1957 Jensen 23273 2,837,618 6/1958 Gildart148-1.5 2,871,377 1/1959 Tyler et a1. 148-173 2,902,350 9/1959 Jenny etal 1481.6 3,012,865 12/1961 Pellin 23-273 3,121,619 2/1964 Scholte148-1.6

OTHER REFERENCES Braun et al.: Article in the Journal of theElectrochemical Society, October 1961, pages 969-973.

Miller: Gradient Freeze Single-Crystal Growth, Compound Semi-Conductors,Reinhold Publishing Corp, New York, vol. 1, pp. 274-279.

DAVID L. RECK, Primary Examiner.

1. AN APPARATUS FOR PRODUCING SINGLE CRYSTAL SUBSTANCES BY THETEMPERATURE GRADIENT FREEZE METHD WHICH COMPRISES IN COMBINATION, ACRYSTALLIZING CONTAINER, A HEAT CONDUCTING ELEMENT SURROUNDING SAIDCONTAINER AND BEING IN HEAT EXCHANGE RELATION THERETO, A HEAT INSULATINGMEMBER SURROUNDING SAID HEAT CONDUCTING ELEMENT TO PREVENT HEATRADIATION FROM THE LONGITUDINAL SURFACE OF SAID CRYSTALLIZING CONTAINER,AND HEATING MEANS SURROUNDING SAID HEAT CONDUCTING ELEMENT TO APPLY AUNIFORM TEMPERATURE GRADIENT ACROSS SAID CRYSTALLIZING CONTAINER, SAIDHEAT CONDUCTING ELEMENT HAVING A TERMINAL END WHICH EXTENDS BEYOND ONETRANSVERSE END OF THE HEAT INSULATING MEMBER, THE TERMINAL END OF SAIDELEMENT HAVING A LENGTH SUFFICIENT TO DRAW HEAT FROM THE HEATING MEANSFOR TRANSMITTING THE HEAT ACROSS THE LENGTH OF THE HEAT CONDUCTINGELEMENT.